1. Field of the Invention
This invention relates, in general, to electrical inductive apparatus and, more specifically, to insulation structures for electrical power transformers.
2. Description of the Prior Art
It is common in electrical inductive apparatus, such as transformers, to construct a magnetic core having rectangularly-shaped leg members with electrical windings, composed of a plurality of layers of conductor turns, and also having a rectangular configuration, disposed therearound. In a two-winding rectangular core and coil configuration, the low voltage winding is normally disposed adjacent the leg of the magnetic core and the high voltage winding is wound around the high-low insulation structure separating the high voltage from the low voltage winding. The high-low insulation structure is formed of a plurality of layers of an electrically insulative sheet material built up to the thickness required to provide adequate insulation between the low voltage and high voltage windings.
As shown in U.S. Pat. No. 3,237,136, which is assigned to the assignee of the present invention, it is common to form cooling ducts in the high-low insulation space to provide additional insulation between the windings of a transformer. The cooling ducts are formed by a plurality of spacer members which are bonded to the low voltage winding and surrounded by layers of electrical insulating sheet material. It is also common to have several layers of cooling ducts in the high-low insulation space to provide fluid flow paths for dielectric fluid therebetween whose dielectric strength provides the additional insulation required for transformers having higher voltage ratings.
The rectangular core and coil type construction has been limited in the past to certain voltage and KVA ratings since larger units have been unable to meet the short circuit withstand requirements. During a short circuit, the low voltage and high voltage windings tend to separate, or move in opposite directions, with the low voltage winding being compressed against the leg of the magnetic core, and the high voltage winding being subject to an outward tensile force. This separation of the low voltage from the high voltage windings during a short circuit causes considerable force to be exerted on the cooling ducts within the insulation space which are bonded to the windings. This force breaks the adhesive bonds holding the spacer members in position or rips the spacer members, which are typically formed of cellulosic material such as pressboard, thereby causing misalignment of the spacers which blocks coolant flow through the ducts and decreases the insulative strength of the high-low insulation structure. Further, the severe forces may cause the spacer members to penetrate the adjacent layers of insulative sheet material, thereby lowering its insulative properties and decreasing the short circuit withstand capability of the transformer.
In a rectangular core and coil configuration, the sides of the rectangular coils are compressed against an insulative barrier, which is compressed against another coil or against the framing structure surrounding the magnetic core and coil assembly. According to the prior art, the insulative barrier consists of solid insulative material, such as pressboard, of suitable thickness which is surrounded on both sides, in high voltage rated apparatus, by a plurality of spaced, vertically-extending spacer members. The spacer members, which form a plurality of cooling ducts therebetween, are disposed in contact with the outermost turns of the adjacent winding assemblies or the support structure surrounding the core and coil assembly. During a short circuit, there is a vibratory force transmitted from one phase to the adjacent phase or to the support structure. This force is sufficient to break the adhesive bonds holding the spacer members to the solid insulating material forming the barrier insulation, thereby resulting in misalignment of the spacer members, which blocks dielectric fluid flow through the cooling ducts in the phase barrier insulation structures and decreases its insulative properties.
Thus, it is desirable to provide an electrical inductive apparatus having improved insulation structures that retain their physical integrity under the forces exerted on the electrical inductive apparatus during short circuit conditions. It is also desirable to provide an electrical inductive apparatus which permits relative movement between the insulation structures and the adjacent windings or barrier insulation to prevent damage to the spacer members forming the cooling ducts within the insulation structures. Finally, it is desirable to provide an electrical inductive apparatus having insulation structures in which the spacer members forming cooling ducts therein are securely held in a vertical orientation throughout the operation of the apparatus.